I should start out by saying that my instructions in this video are not very clear at all, so I'll explain how to do it in more detail here.
First of all, make sure that all of the main DCO functions are muted, including the noise and sub sliders.
In the VCF section, turn the cutoff frequency right down and put the resonance right up. Set the KYBD (keyboard follow) slider to full and the LFO and ENV sliders to zero.
Make sure that VCO is set to gate, or if you want to use an envelope, set it so that the attack and release are at zero and the decay and sustain are at full.
Now, hold down middle C and gradually raise the filter frequency slider. The keyboard should begin to make a sound. Keep moving the slider up until the tone you hear is just below middle C. Lower the RES (resonance) slider slightly to sharpen the tone.
If you've done it right you should have a sine wave at roughly middle C. Check it against a tuner, or another keyboard.
It's not perfect, but it's a neat little trick. I said in the video that you could do "all kinds" with it - which in hindsight was a bit of an exaggeration. You can apply the LFO to it, or use the envelope to create Atari-like laser gun sounds. If you invert the envelope you get some pretty interesting sounds too.
Not sure if this works with other analogue synthesizers, I'd be interested in finding out if somebody's willing to try this.